Not Applicable
This invention relates generally to small footprint transformers equipped with heat dissipators and, more particularly, to improved transformer constructions adapted to the more efficient cooling arrangements for dissipating heat generated in the winding structure of power transformers.
Transformers, as most electric apparatus and equipment, do not have specific rating: their load carrying capacity is limited only by their temperature. In transformer windings, due to their resistance, losses are generated proportionally to the square of the load currents and eddy currents, warming up the windings. Their temperature, however, depends on the efficiency of the cooling arrangement used for removing the generated losses.
In the present practice, natural convection plays the largest role in cooling via the surface of the winding. Tubular windings are in use almost exclusively. If the outside surface of the winding does not provide sufficient heat transfer, the present practice is to create cooling ducts between winding layers by separating the layers with spacers. These ducts are not very efficient, because the cooling medium moves slowly in narrow spaces, and warms up considerably before finally exits at the top of the duct. Consequently, the temperature at the top portion of the winding is much higher than at the bottom portion.
Including wider ducts increases the mean turn length of the winding. Thus the weight of the winding also increases, and the losses. Using longer core legs and longer windings to increase the cooling surface, but the losses further increase. Deviating the configuration more from the optimum format, the toroidxe2x80x94which has the minimum material content, but inferior cooling surfaces for natural convection creates this increase. Generally, a large part of the gain expected from enlarging the cooling surfaces of the winding is canceled by increased weight and losses.
Several attempts are documented in the prior art to improve the cooling process by including highly heat conductive metal sheets into windings. None of the prior art uses a dissipator displaying features of the present invention and achieves significant improvement except one: U.S. Pat. No. 3,659,239 to Marton, Apr. 25, 1972. This patent, however, limits the use of heat dissipators to tubular layer-wound winding structures mounted on vertical core legs. The layers of the windings are interleaved with contiguous portions of dissipators wound into the windings alternating with the winding layers. A louver-like structure is prefabricated on an extended portion of the dissipator sheets, and arranged outside the winding. The louver-like structures are cut into segments containing a group of fins. The segments are bent into horizontal position disposed in planes at both ends of each layer. The segments build up several levels of fins. The major surfaces of the fins are oriented close to vertical. With this orientation the channels are wide, and the resistance to the flow of the cooling medium is small.
With the heat dissipators in this configuration, substantial improvement can be achieved: Keeping the costs and materials the same, the winding losses and temperature rise can be reduced. These values are less than half of the conventional values. Keeping the same losses, 30% winding material, and 12% core steel can be saved with 15% less temperature rise.
Between 1968 and 1976, four small companies in a row manufactured about 3000 units with tubular heat dissipators according to this patent. These units are still in flawless operation. This small scale production has been discontinued only because of lack of interest in energy saving, lack of honest cooperation between partners, unfair competition, and lack of adequate working capital.
During the elapsed 32 years, this technology has been offered five times to every U.S. transformer manufacturer. All of them rejected it. In 1978, it was submitted to the invention evaluation program sponsored by the U.S. Department of Energy. Two independent engineering companies evaluated it with positive recommendations. In 1980, the Department of Energy still refused to offer meaningful support. Thus, in the past twenty-five years, the substantial improvements introduced by this technology remain unused.
All present transformer production uses the conventional 100-year-old technology.
This presently unused technology of U.S. Pat. No. 3,659,239 uses layer-wound tubular windings with wound-in heat dissipators. It has several drawbacks. Some of the drawbacks emerge in the production. In this process the dissipators are incorporated into the winding structure at the winding operation. First, the dissipator sheet is bent to follow the curvature of the designated winding layer, wrapped in the proper insulating sheet and placed over the layer. After securing the heat dissipator in its correct position, the next layer is wound over it. Special attention is required to wind very tightly to eliminate any gaps between the layers and the dissipators to keep the internal temperature gradient low. Winding tightly is a slow process.
Tubular winding structures generate leakage flux inside windings; this flux is oriented parallel to the axis of core legs. This flux orientation makes heat dissipator application very difficult when the winding is built up from discs. In flat contiguous dissipators, heavy eddy-currents would develop. To prevent this problem by splitting up the inserted portion of the dissipator into narrow sections, the tooling becomes prohibitively expensive, and the assembly gets complicated. Furthermore, the method described in the prior art cannot be used with dissipators having longer fins. The contour of the windings has a large variety of curvatures, and a separate tool would be required for every different curvature. Thus, the application of heat dissipators in tubular windings built up from discs is limited to short fins, usable only in liquid cooling. Considering the expensive tooling costs and the additional labor costs this version requires, dissipator cooling for discs in tubular winding systems is not economical.
Further drawbacks in layer wound windings become apparent after removing the completed winding from the winding machine. The several levels of louver-like structures on the curved extensions are hand-cut into uneven smaller segments. This type of subdivision is necessary to allow the 90 degree outward bending of the cut-up irregular fin groups. The cut up segments are bent into their final horizontal radial position. Several levels are built up on both ends of the vertical tubular winding.
The combined work of tight winding, dissipator implantation, and the subsequent cutting and bending operations of the dissipators require additional skilled labor time and extra care. Due to the uneven hand-cutting of the bent louver-like structure, the finished transformers don""t have a smooth professional appearance. This aspect tends to diminish the acceptability of the product for some customers.
Another shortcoming emerged in the practice. When during assembly or cleaning, the fin segments have been bent up and down three times, they have the tendency to break off. This failure can be remedied only by replacing the winding. After impregnation, there is no remedy possible.
When building transformers with higher kVA rating, the efficiency of the dissipator arrangement diminishes. This occurs due to larger internal temperature gradients developing along the longer layers. There is difficulty of accommodating more levels of louver-like structures crowding at both ends of the windings. This difficulty can be alleviated by assigning extra space along the leg for the louver-like structures. This can be done by interrupting the winding, subdividing it into sections. This solution leads to longer legs, thus heavier units andincreased losses. If the interruption is applied only to the upper layers, some of the louver-like structures have to be cut into segments and bent up on the winding machine. Continuing the winding with the bent-up segments may cause injury to the fin segments, or to the winder.
The subdivided arrangement leads to a larger number of fin segment levels. The cooling gradually diminishes on each subsequent higher level. The upward moving flow gets more and more preheated. To avoid the building up of peaks in the temperature of the winding, more heat dissipators need to be added to sections on higher levels.
In larger transformers, where winding must be subdivided into two or more sections along the vertical core leg, the effect of preheated cooling medium and longer legs is more and more pronounced. In addition, the connections of the multiple segments of the windings become difficult to accommodate in the limited space left open by the fin segments. Ultimately, these difficulties limit the size of the units that is economically feasible with dissipator-cooled layer-wound windings presented in the prior art.
The present invention offers methods for building transformers with the following substantial improvements:
(1) Compared to the transformer technology presently in general use:
(1.1) Building for standard specification, production costs can be reduced up to 40%,
(1.2) As an alternative, keeping the same production costs, the losses and the temperature rise can be reduced by close to 60%.
(1.3) All units have reduced floor space requirement.
(2) Compared to the only relevant, but presently unused prior art:
(2.1) The production is simpler: it requires less time and less skilled labor, thus it reduces production costs.
(2.2) The difference between average and peak temperatures is reduced to a few degrees.
(2.3) The lower peak temperature leads to higher rating with the same active material content.
(2.4) All units have reduced floor space requirement.
(2.5) There is no size limit for the application of the new technology.
(2.6) Any cooling medium (air, SF6, oil, etc.) can be used.
(2.7) All units have well organized, attractive appearance.
In view of the foregoing, several objects and advantages of the present invention are outlined in the following paragraphs.
Winding structures on transformer legs are superimposed, one over the other. This configuration coupled with close to square windows leads to smaller floor space requirement.
Its winding structure can be assembled using a number of identical disc coils. These discs can be produced using multiple winding techniques and saving labor time and production costs. No dissipators are involved in the winding operation.
It applies plane dissipators inserted at the end of the assembly operation into the discs. This procedure is simple and quick.
The plane dissipators have unobstructed access to fresh cooling medium. Thus, the peak temperature of the winding is close to the average, leading to higher ratings for units with the same active material content.
The ratings of the transformers have no limitations. The disc coils with or without dissipators can be built for any rating with no problems.
The disc coils with or without dissipators can be built for any cooling medium (air, SF6, oil, etc.). The dimensions of the fins need to be adapted to the convection potential of the selected medium.
The coils line up on the core leg with their inserted dissipators in a row having the same dimensions; they offer a well organized, attractive appearance.
The invention achieves one of its objects by offering the possibility of building transformers with windings composed of identical disc coils. These disc coils can be multiple wound between flanges on the same machine, saving labor time. There is no need for tight winding: no heat travels between layers. Subsequently, the coils can be impregnated without removing them from the mandrel. The solid disc coils can be easily and safely assembled on a core tube.
The invention achieves its additional objects by applying louver-like heat transfer surfaces between the plane heat dissipator and the cooling medium. The louver-like heat transfer surfaces of the plane dissipators are contiguous extended portions of the sheet. The first step is splitting the extended portion into fins. Next, each fin is spaced apart of its original position by a selected amount of displacement and/or rotation in the fabricating process. These plane dissipators placed between disc coils at the assembly operation without any change. There is no need for hand-cutting into fin segments. There is no bending, and no chance of breaking off by repeated bending.
The invention further achieves its objects by building up the winding from a multiplicity of disc-like coils with relatively short radial dimension. Thus the heat, picked up by the dissipator, travels only along its short radial dimension before reaches the louver structure. This arrangement leads to minimum internal temperature gradient. The leakage flux also oriented in radial direction between the primary and secondary windings. Since both the dissipators and the leakage flux have radial orientation, there is no interference between them.
The invention further achieves its objects by offering a way to build transformers for larger kVA ratings with a larger number of discs. These discs have a larger circumference, without a significant increase of the radial dimension. Consequently more and longer dissipators can be interleaved with the larger discs without diminishing the efficiency of the heat flow. There is no size limit for the application of the new technology.
This feature is especially pronounced when the discs are arranged on a horizontal core leg and interleaved with vertical plane dissipators. In this arrangement every part of the winding has access to fresh non-preheated cooling medium minimizing the temperature peaks in the winding.
The novel features of the invention are set forth with particularity in the appended claims. The invention will be best understood from the following description when read in conjunction with the accompanying drawings.